The aid agency believes $50bn a year (£30bn) is needed to help developing countries cope with the impacts of global warming including droughts, floods, storms and rising sea levels.
And it says the money must be provided in addition to the 0.7% of GDP developed nations have pledged as aid to improve the lives of people in some of the world’s poorest countries – or efforts to tackle poverty will stall.
A report by Oxfam warns that diverting $50bn from existing aid pledges to fund climate measures would lead to the death of 4.5 million children, while 75 million fewer youngsters would be likely to go to school and 8.6 million fewer people would have access to HIV/Aids treatment.
It could prove a major setback to efforts to meet the Millennium Development Goals which aim to end hunger and poverty and boost education, health, gender equality and environmental sustainability by 2015, the report warns.
Oxfam said it was already seeing people going without food, pulling their children out of school or selling livestock to pay for debts caused by failing crops and other climate-related problems.
According to the aid agency, just three countries including the UK are in favour of additional funding for climate measures – and the issue could prove to be a deal breaker in the upcoming crunch talks aimed at agreeing global emissions cuts in Copenhagen in December.
A failure by developed countries to address the problems surrounding adaptation funding has led to distrust between the two sides and could undermine efforts to secure a deal to cut emissions.
Oxfam is also concerned that a Conservative government in the UK would divert existing aid provisions to pay for measures such as flood prevention and the introduction of drought-resistant crops.
Barbara Stocking, chief executive of Oxfam Great Britain, said: “Forcing poor countries to choose between life-saving drugs for the sick, schooling for their children or the means to protect themselves against climate change is an unfair burden that will only exacerbate poverty.
“Stealing money from tomorrow’s schools and hospitals to help poor people adapt to climate change is neither a moral or effective way of rich countries paying their climate debt.
“Funds must be increased, not diverted,” she said.
Oxfam wants to see a carbon market in which rich countries have to buy allowances to cover national emissions under a new global deal to slash greenhouse gases, with the money going towards paying for adaptation measures.
The scheme, similar to one which has been proposed by the Norwegian government in advance of Copenhagen, would avoid the “familiar problem” of developed countries failing to meet aid promises, the Oxfam report’s co-author Robert Bailey suggested.
A spokeswoman for the Department for International Development (DfID) said: “Climate finance will be one of the most important and most challenging issues to be addressed over the coming years and that is why the UK are leading the way by offering new investment in addition to our existing aid commitments.
“In June the UK became the first country to publicly address the issue with the proposal for an annual $100bn global fund, to help developing countries both prepare for the impacts of climate change and build for a low-carbon future.”
The shadow international development secretary, Andrew Mitchell, said: “We must tackle both the causes and the consequences of global climate change.
“As well as setting the framework for carbon markets, international agreements will be key to establishing additional support for adaptation.
“We believe that Britain must work towards an ambitious global deal at Copenhagen that will limit emissions and see substantial financial resources made available for adaptation.”